A single push-pull metal-oxide-semiconductor field-effect transistor (MOSFET) pair has typically been used to drive a signal on and off. Unfortunately, the impedance of the push/pull pair is dependent upon variations in fabrication process, voltage and temperature conditions.
To address this problem, one approach was developed in which signal drivers were divided into multiple (e.g., five) separate drivers. The current outputs of the signal drivers were aggregated to generate a desired signal output. Factors of two distinguished the currents of each of the signal drivers, which allowed a binary code to drive the signal drivers to produce the desired output current for any given fabrication process, voltage and temperature. However, this “N separate signal driver” approach had its own problems. For instance, the impedance of the signal driver as a whole tended to change momentarily and dramatically (giving rise to what is colloquially called a “glitch”) as the separate signal drivers switched on and off when compensating for changes in fabrication process, voltage and temperature.
The switching speed, or slew rate, of the driver can be adjusted by further subdividing the driver into M equal sections (e.g., 4) and delaying the switching of each. However, grounding capacitors were required at the output of the driver to help slow down edges. Unfortunately, these grounding capacitors occupied valuable area on an integrated circuit (“IC”) chip.
An alternative, “digital thermometer” approach to a signal driver was also developed. In this approach, the outputs of an array of separate drivers (perhaps 60 to 100) of equal current capacity were coupled in parallel and enabled or disabled to produce the desired output current for any given fabrication process, voltage and temperature. Unlike the binary coded driver, the thermometer coded driver does not glitch when changing the current drive strength when compensating for changes to the fabrication process, voltage, and temperature.
The slew rate control mechanism applied to the binary coded driver, where the driver is further subdivided, is not practical to thermometer coded drivers because of the large number of resulting circuits and lack of area on the IC to implement the scheme.
Process corners are parametric extremes, such as physical dimension, uniformity and chemical composition, evident in a given population of fabricated ICs. The process corners define the envelope within which the population of ICs behaves. The corner of behavior that leads to a fastest slew rate occurs at a high voltage, low temperature corner of operation and is therefore known as the “fast” or “strong” corner. Likewise, the corner of behavior that leads to the slowest slew rate occurs at a low voltage, high temperature corner and is therefore known as the “slow” or “weak” corner. Also, the strength of the silicon itself has an affect on the determination of whether it is the “fast” or “slow” corner.
In circuit design, one should design for the worst-case scenario, which in this context means proper operation even at the process corners. A circuit designer therefore needs to take into account the various possible slew rates of an IC in either its “strong” or “weak” corner.
Accordingly, what is needed in the art is a signal driver that has a selectable aggregate slew rate to compensate for varying fabrication process, voltage or temperature conditions.